1. Field of the Invention
The present invention relates to a technique for efficient spectrum utilization in mobile radio systems using space diversity and, more particularly, to a technique in cellular mobile radio systems wherein mobiles in an associated cell communicate with base stations of that cell using separate primary channels within a major portion of the overall system frequency spectrum when a mobile is not experiencing interference from adjacent cells above a predetermined interference level and are switched to a separate secondary channel within a selected section of the remaining minor portion of the overall system frequency spectrum when a mobile experiences interference above the predetermined interference level.
2. Description of the Prior Art
In a Rayleigh fading environment with a signal and interferences coming from all directions, a space diversity receiver cophased to the desired signal provides coherent combining of the desired signal and incoherent combining of the interferences. In a non-interference environment, space diversity combining smoothes out the amplitude fluctuations of the received signal. For example, in a Rayleigh fading environment, to insure that a single branch receiver remains above some threshold 99.9 percent of the time requires that the mean received power be about 30 dB above this threshold, whereas this same threshold can be exceeded the same amount of time with 26 dB less transmitted power if 4 space diversity branches are employed. These same general remarks hold true in the case of interference. Once the number of diversity branches reaches 10 or so, envelope fluctuations in the received signal essentially disappear and signal-to-interference increases in proportion to the number of diversity branches.
The provision of mobile telephone service to a large number of users is most efficient when frequencies are reused through cellular building blocks. Hexagonal cells often are chosen for design purposes because they are fairly good approximations to circles. Also because hexagons tesselate a plane, frequency reuse is achieved in an orderly way. An article "Advanced Mobile Phone Service: The Cellular Concept" by V. H. MacDonald in The Bell System Technical Journal, Vol. 58, No. 1, January 1979 at pages 15-41 describes a mobile radio-telephone system which can include hexagonal cells having three alternate corners equipped with base stations including 120 degree directional antennas. Channel sets are defined and distributed among the various cells to keep co-channel and adjacent-channel interference within acceptable bounds. With reference to hexagonal cellular regions, see also U.S. Pat. No. 3,582,787 issued to J. J. M. Garches et al on June 1, 1971 and U.S. Pat. No. 4,128,740 issued to V. Graziano on Dec. 5, 1978, the latter patent using sectoral horns for cell communication.
Spread spectrum techniques with frequency hopping have been proposed as an alternative method for mobile communication in recent years. Conceptually, the success of spread spectrum frequency reuse techniques is due to their ability to cope with cochannel interference by the combining, in one form or another, of frequency diversity channels. Additionally, frequency diversity is a wellknown, effective technique to combat Rayleigh fading. However, it appears that any frequency diversity scheme must pay a price in the inevitable reduction of spectrum efficiency. If cost and receiver complication are not issues, space diversity at both the mobile and base offers an opportunity of providing very high spectral efficiency as well as reducing fading.
The problem remaining in the prior art is to provide a mobile radio communication system which provides improved spectrum efficiency and can provide a significant advantage against shadow fading.